This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-186535, filed Jun. 30, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an optical recording medium, a reproducing method, and a reproducing apparatus and, more particularly, to an optical recording medium having a structure in which a plurality of recording layers are stacked, and a reproducing method and apparatus for the medium.
Optical recording media such as optical disks are used in various forms. For example, as external storage media of computers and as music media, CDs (Compact Discs) such as CD-ROM (read only), CD-R (write-once), and CD-RW (rewritable) are extensively used. Also, DVDs (Digital Versatile Discs) are becoming popular as optical recording media having larger capacity and are used in video recording, in computer storaging, and the like in recent years. DVDs also have diverse types: DVD-ROM (read only), DVD-R (write-once), and DVD-RAM (rewritable).
Of these optical recording media, a read only medium, for example, has a recording layer in which pits having a depth xc2xc the wavelength of a laser beam are formed as recording marks. When this pit is irradiated with a laser beam, the phase of reflected light from the pit shifts by xc2xd from that of reflected light from the periphery of the pit. Consequently, these reflected light components produce interference. Therefore, compared to the case in which a portion having no pit is irradiated with a laser beam, the intensity of reflected light detected by a photodetector lowers. A read only optical recording medium uses this principle in reproducing recorded information.
DVDs have a structure in which a pair of disks each having a thickness of 0.6 mm are adhered. A DVD in which recording layers are formed on both opposing surfaces of these disks is called a dual-layered DVD. Of such dual-layered DVDS, a DVD that allows information recorded in a pair of recording layers to be read out from one side is called a single-sided, dual-layered DVD.
FIG. 1 is a sectional view schematically showing a conventional single-sided, dual-layered DVD. As shown in FIG. 1, this conventional single-sided, dual-layered DVD 101 has a structure in which a substrate 102 having a recording layer 104 on one principal surface and an opposing substrate 103 having a recording layer 105 on one principal surface are adhered via an interlayer 106 such that the recording layers 104 and 105 oppose each other. Note that the recording layer 104 is a semitransparent film and the recording layer 105 is a reflecting film.
Information recorded in the recording layer 104 of this conventional single-sided, dual-layered DVD 101 is read out by sending a laser beam 107, focused on the recording layer 104, from the side of the substrate 102, and detecting the intensity of the reflected light by a photodetector. Although reflected light from the recording layer 105 is also detected, the focal depth of this layer beam 107 is small, so the intensity of the reflected light from the recording layer 105 is much lower than that of the reflected light from the recording layer 104. Accordingly, only the information recorded in the recording layer 104 can be selectively read out.
On the other hand, to read out information recorded in the recording layer 105, a laser beam 108 focused on the recording layer 105 is sent from the side of the substrate 102. As in the above case, only the information recorded in the recording layer 105 can be selectively read out.
In this prior art as described above, in reading out information recorded in one of the recording layers 104 and 105, information recorded in the other is prevented from becoming noise by using the small focal depth of the laser beam used to read out information. This allows read-out of information recorded in the recording layers 104 and 105. Hence, in this prior art it is necessary to move the focal position of the laser beam.
To move the focal position, a mechanism for moving a lens must be incorporated into an optical disk drive. This significantly increases the time and cost necessary to manufacture the optical disk drive of this prior art. Also, when the focal position is thus moved, reading out recorded information naturally requires a long time. Accordingly, it is difficult for this prior art to realize a high read rate.
It is an object of the present invention to make it possible to read out information recorded in a single-sided, multilayered optical recording medium at high speed.
It is another object of the present invention to reduce the cost and time required to manufacture a reproducing apparatus for a single-sided, multilayered optical recording medium.
According to the first aspect of the present invention, there is provided an optical recording medium using a reflectance difference to reproduce recorded information, comprising a first recording layer, an interlayer provided on one principal surface of the first recording layer, and a second recording layer provided on the interlayer, wherein the interlayer guides converged light irradiating the first recording layer to the second recording layer such that a beam diameter of the light on the second recording layer is substantially equal to a beam diameter of the light on the first recording layer.
According to the second aspect of the present invention, there is provided an optical recording medium using a reflectance difference to reproduce recorded information, comprising a first recording layer, an interlayer provided on one principal surface of the first recording layer and made of a third-order nonlinear optical material, and a second recording layer provided on the interlayer.
According to the third aspect of the present invention, there is provided an optical recording medium using a reflectance difference to reproduce recorded information, comprising a first recording layer, an interlayer provided on one principal surface of the first recording layer, and a second recording layer provided on the interlayer, wherein the interlayer comprises first and second portions juxtaposed on the first recording layer, and a refractive index of the second portion is higher than that of the first portion.
According to the fourth aspect of the present invention, there is provided a reproducing method of an optical recording medium comprising a first recording layer, an interlayer provided on one principal surface of the first recording layer, and a second layer provided on the interlayer, comprising the steps of irradiating the first recording layer with converged light, and detecting the intensity of reflected light produced by irradiation of the converged light, wherein the interlayer guides the converged light irradiating the first recording layer to the second recording layer such that a beam diameter of the light on the second recording layer is substantially equal to a beam diameter of the light on the first recording layer.
According to the fifth aspect of the present invention, there is provided a reproducing apparatus of an optical recording medium having a structure in which a plurality of recording layers are stacked, comprising an optical recording medium comprising a first recording layer, an interlayer provided on one principal surface of the first recording layer, and a second recording layer provided on the interlayer, a light source for irradiating the first recording layer with converged light, and a detector for detecting the intensity of reflected light produced by irradiation of the light, wherein the interlayer guides the converged light irradiating the first recording layer to the second recording layer such that a beam diameter of the light on the second recording layer is substantially equal to a beam diameter of the light on the first recording layer.
The term xe2x80x9creflectancexe2x80x9d means a value actually measured by irradiating a reflector with light, when it is used like xe2x80x9cthe difference between reflectancesxe2x80x9d and xe2x80x9cdetect the reflectancexe2x80x9d. That is, when this is the case the reflectance not only changes in accordance with the wavelength of the light used and the material and thickness of the reflector but also is influenced by the shape of the reflector. Accordingly, xe2x80x9cdetect the reflectance of that portion of an optical recording medium, which is irradiated with converged lightxe2x80x9d is equivalent to xe2x80x9cdetect the energy or intensity of reflected light produced when the first recording layer is irradiated with converged lightxe2x80x9d. Also, when the term xe2x80x9creflectancexe2x80x9d is used in connection with a single thin flat film like xe2x80x9cthe reflectance of a first recording layerxe2x80x9d, this reflectance is uniquely determined only by the wavelength of the light used and the material and thickness of the thin film.
In the present invention as described above, converged light irradiating the first recording layer is guided to the second recording layer such that the beam diameter of the light on the first recording layer is equal to the beam diameter of the light on the second recording layer. This means that when this converged light is focused on the first recording layer, the light is also focused on the second recording layer at the same time. In the present invention, therefore, unlike the prior art, reflected light detected by the photodetector can contain light components reflected by the first and second recording layers respectively with sufficient intensities. Hence, by appropriately setting the light transmittance and the like of the first recording layer, it is possible to read out both information recorded in the first recording layer and information recorded in the second recording layer at the same time.
In the present invention, readout information is usually separated in accordance with information recorded in the first recording layer and information recorded in the second recording layer. However, information read out by the above method need not be separated. For example, when both first and second recording layers are used to record one information, i.e., when a recording mark pattern formed on the first recording layer and a recording mark pattern formed on the second recording layer are related, it is unnecessary to separate readout information.
In the present invention as described above, it is possible to simultaneously read out information recorded in the first recording layer and information recorded in the second recording layer without moving the focal position of converged light. Accordingly, the present invention can read out information recorded in a single-sided, multilayered optical recording medium at very high speed. Also, the present invention obviates the need for a mechanism for moving the focal position of converged light. This can reduce the cost and time required to manufacture a reproducing apparatus for a single-sided, multilayered optical recording medium.
In the present invention as described above, the interlayer guides converged light irradiating the first recording layer to the second recording layer such that the beam diameter on the second recording layer is substantially equal to the beam diameter on the first recording layer. This interlayer is made of, e.g., a third-order nonlinear optical material. The interlayer can also have first and second portions juxtaposed on the first recording layer, and the refractive index of the second portion can be higher than that of the first portion.
In the present invention, a reflectance R1 of the first recording layer and a reflectance R2 of the second recording layer preferably satisfy:
0.1 less than R1 less than 0.4
0.5 less than R2xe2x89xa61, and
R2xe2x89xa0R1/(1xe2x88x92R1)
When this is the case, the reflected light intensity or reflectance detected changes between four values, and sufficient reflected light intensity is obtained. So, this is advantageous in separating readout information.
In the present invention, the first and second recording layers are preferably different from each other in at least one of a recording density and a recording method. In this case, readout information can be separated by using this difference.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.